A white LED emits a white light by mixing luminescences from a near ultra violet or blue LED and a phosphor. Conventionally, development and research have been vigorously made of the white LED as an LCD backlight light source for a small size portable device or the like, while expansion to an illumination application thereof as a next-generation application advances.
In the backlight application or the like, a so-called pseudo white has been widely used in which a blue LED and an YAG:Ce3+ are used in combination. However, the white light obtained by this combination has a problem that the color rendering property is low when used for illumination due to the lack of red components. In order to improve this point, a white LED with the use of a blue LED along with a green or yellow phosphor and a red phosphor is proposed. In addition, as a white LED having a higher color rendering is also proposed a white LED with the use of a near ultra violet to violet LED along with blue, green and red phosphors in a combined system.
As an example of the red phosphor used for these white LEDs, nitride phosphor such as CaAlSiN3:Eu or (Sr, Ca)AlSiN3 (e.g., refer to Patent Literature 1 or 2), or a sulfide phosphor such as CaS:Eu, SrS:Eu or (Ca, Sr)S:Eu (e.g., refer to Patent Literature 3) is proposed.
Although the nitride phosphor has a high performance, a producing step is required of performing annealing at a high temperature of around 2,000° C. under a nitrogen pressurized atmosphere, which makes the production difficult and requires a special facility. In addition, although the sulfide phosphor is relatively easily produced, there is a problem in that the production involves generation of a bad smell, or corrosion of a wiring material such as Ag or Cu caused by sulfur generated through decomposition.
In addition, these nitride phosphor and sulfide phosphor have an excitation spectrum extending to a long wavelength side, so that when a white LED is prepared by mixing them with a yellow to green phosphor, there is also a problem in that they are easy to reabsorb light emitted from the green to yellow phosphor and then emit light, that is, a so-called multistage excitation is easy to occur. When such red phosphors are used and mixed with a green or yellow phosphor and a blue excitation is made, there easily arises unevenness in color, or deterioration of the luminous efficiency of the white LED. In order to reduce the effect of such a multistage excitation, a structure in which fluorescent layers are layered or separated is proposed. However, there is a problem in that a producing step for a white LED is complicated.
As one of phosphors other than the nitride phosphor and the sulfide phosphor, a europium (Eu)-activated alkaline earth metal silicate phosphor is known. For example, (Sr, Ba)2SiO4:Eu is well-known. Such an alkaline earth metal silicate phosphor is widely used, because of the characteristics that the production is relatively easy but does not require a special producing facility, and adjustment of the emission wavelength is possible in accordance with the Ba/Sr ratio.
However, in such alkaline earth metal silicate phosphors, the one having an emission peak wavelength of more than 600 nm is not known. A conventional alkaline earth metal silicate phosphor has too short wavelength to be used as a red phosphor (e.g., refer to Non Patent Literature 2).
In addition, in Patent Literature 4 is disclosed a phosphor having a composition of (Srx, Bay, Caz, Euw)2SiO4:Eu, which emits light having a long wavelength of 600 nm or more by a blue excitation. However, it is defined that, in order to prevent an increase in the moisture absorption, the adding amount of a crystal grower is restricted to 0.01% by weight or more but 0.3% by weight or less with respect to a whole base powder. In such a case, there are problems in that not only the crystal growth occurs insufficiently to produce a practically sufficient luminance, but also the obtained phosphor has a so deformed shape that, when the phosphor is mixed in resin for the production of a white LED element, there easily arises ununiformity or unevenness.